The structure and magnetic properties of the Bi2Sr2Cu1-xMxOy (M=Co and Fe) materials were studied. The limits of solid solution formation are at x=0.5 for the Fe system and x=1 for the Co system. Crystals of the new Bi2Sr2CoOy phase were grown and the structure established by x-ray crystallography. The subcell is the same as that of the 10-K superconductor, Bi2Sr2CoOy, but the superstructure is different, as it exhibits a commensurate modulation of periodicity 4 instead of 5. Extra oxygen is accommodated in the Bi layers, as in Bi2Sr3Fe2Oy, and the structure of the Bi-O layers can be described as 50% rocksalt type and 50% oxygen-deficient perovskite for x=1, but with disorder at the oxygen positions. The formal valence of Co in this compound is about 2.50.2 as deduced from structural and chemical measurements, whereas Fe adopts the oxidation state +3 as deduced by Mössbauer measurements. Bi2Sr2CoOy is an antiferromagnetic insulator with the spins lying within the CoO2 sheets and the antiferromagnetic ordering temperature (TN) is sensitive to processing conditions and composition changes. The high anisotropy of the susceptibility suggests that Bi2Sr2CoOy may be an Ising or XY antiferromagnet.
ASJC Scopus subject areas
- Condensed Matter Physics